The best I can offer you is
this.
It is an alphabetically sorted list of pretty much every file extension you're going to be likely to encounter on a Rockboxed device, which I created for my own uses many moons ago. I simply stripped from it my custom icon assignments so you can do as you please with it.
Remember 0-31 for the viewers icons, and *0-*32 for the main icon set and remote icon set, - for "no icon".
The way the .icons file works doesn't allow for
".?", ".??", or ".???"
nor
".*", ".**", or ".***"
to be treated as unknowns, but rather, it assumes them to be the literal extension.
Edit this file as you desire, remember it must be saved in the /.rockbox/icons/ directory as ".icons" to function, I will more than likely put it on the wiki at some point as a base for others to do the same. I hope this saves you some time and hassle. I am sorry it doesn't work the way you want it to.
[Saint]